Exposing structural materials to aggressive environments under steady or cyclic stress can give rise to damage in the form of cracking. This is often referred to as "stress corrosion cracking" or "corrosion fatigue." Stress corrosion cracking of structural materials in aggressive environments is a continuing problem in many industries. The nuclear industry in particular continues to encounter this problem where the structural materials operate under sustained or cyclic stress in the presence of high temperature water.
Damage in the form of stress corrosion cracking, or other stress/environment-induced cracking, hereinafter referred to collectively as stress corrosion cracking, is of much greater concern in industry than damage such as that caused by uniform corrosion. Uniform corrosion results in a predictable service life for components. On the other hand material failures due to stress corrosion cracking are not easily predicted and are generally significant in nature.
U.S. Pat. No. 4,677,855 issued to Coffin, Jr. et al., the subject matter of which is hereby incorporated by reference, sets out problems which industry in general, and the nuclear industry in particular, faces in attempting to predict the onset of or susceptibility of particular structural components to stress corrosion cracking. In general, the performance of structural components is predicted in advance from information on the expected loadings and resulting stress from these loadings. Although these predictions are Sufficiently accurate to predict service performance, it is difficult to predict the lifetime of such performance due to uncertainty in the environmental conditions and the influence thereof on the stress corrosion cracking which results therefrom.
An example of the uncertainty of lifetime predictions for structural materials is the stress corrosion cracking which has been found to occur in stainless steel piping used in the nuclear industry. Although designs for new plants attempt to compensate for this phenomenon, it is desirable to monitor and assess the extent of damage in plants which have been operating for a number of years to help predict their lifetimes and possibly extend their lifetime. Methods for assessing the state of damage have been directed toward monitoring the aggressive environment. The water chemistry is measured to determine factors such as resistivity, electrochemical potential, oxygen level and impurity levels. Such measurements are indirect. No direct measurement is made of the effect this water chemistry has on crack growth in the structural materials during plant service. Therefore, the extent to which the lifetime of the structural material is extended by varying operating conditions are unknown.
Methods for directly measuring crack growth in specimens removed from their environment have been disclosed over the years, including a variety of monitoring systems using visual and voltage potential drop methods. An example of an apparatus for applying a spreading pressure to a slotted fracture specimen to induce cracking of the fracture specimen is disclosed by U.S. Pat. No. 4,075,884 issued to Barker. The Barker patent discloses applying a spreading pressure to the fracture specimen by inflating a pressure bag installed within the slot. Pressure for inflating the pressure bag is generated by a complicated specimen loading machine with a pressure chamber in which pressure is increases by the turning of a screw.
The apparatus disclosed in the Barker patent suffers from several disadvantages making it inoperable in the hostile environment of a nuclear reactor. First, the pressure bag may be adversely affected by the high temperatures, pressures and neutron flux in nuclear reactors. Second, the assembly for providing the pressure for the pressure bag is a complicated system involving a lot of equipment that is not useful in hostile environments and the limited space (approximately 18 inches) reserved for reactor surveillance specimens in a nuclear reactor. Due to the temperature, pressure, neutron flux, and space limitations, an electrical control system is more desirable and practical than a gas or fluid control system.
It was not until the method disclosed in the Coffin, Jr., et al. patent, that the industry was provided with the capability to accurately assess crack growth of plant structural components through voltage potential drop methods by disclosing a reasonably accurate way to relate voltage measurements to crack size. This method was improved upon by U.S. Pat. No. 4,924,708 issued to Solomon et al., the subject matter of which is hereby incorporated by reference.
The methods disclosed by the Coffin, Jr., et al. and Solomon et at. patents both utilize the same type of fracture specimen. Fracture specimen 1, disclosed by the Coffin, Jr., et at. and Solomon et al. and shown in FIG. 1, is of a double cantilever beam (hereinafter DCB) configuration. A voltage is applied to the ends of beams 2, 3 at points 4, 5, and the voltage drop across crack 6 is measured by pairs of probes 7, 8; 9, 10; 11, 12 positioned on each side of beams 2, 3. The changes in the voltage drop over time are used to calculate the growth of crack 6. When a current is caused to flow through fracture specimen 1 perpendicular to crack 6, the potential difference between two points located on opposite sides of the crack will increase as the size of crack 6 increases. Stress is placed on crack 6 by a spreading force acting to spread beams 2, 3 apart. The spreading force is created by stress creating member 13 that is positioned between beams 2, 3.
This DCB configuration suffers from two disadvantages. First, as crack 6 grows, the compliance of beams 2,3 increases. Since the size of stress creating member 13 is static, the stress placed on crack 6 by stress creating member 13 decreases as crack 6 grows. Second, the use of stress creating member 13 allows only for continuous stress on crack 6 as opposed to cyclical application and removal of the stress, i.e., fatigue stress, to more accurately reflect operating conditions.
This invention overcomes these disadvantages by applying a substantially constant stress to the crack and allowing for cyclical application and removal of stress to the crack. Moreover, the present invention is capable of operating in the hostile environment of a nuclear reactor.